Do not required .git in the build context

- export DOCKER_GITCOMMIT in the Makefile
- prioritize DOCKER_GITCOMMIT against the `git` command in
  `./hack/make.sh`
- Also add `integration-cli/bundles` to gitignore (it's generated when
  using integration-cli shell)

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

.dockerignore

@@ -1,4 +1,5 @@
 bundles
+integration-cli/bundles
 .gopath
 vendor/pkg
 .go-pkg-cache